In bacteria, protein translocation across the membrane is stimulated by membrane-integrated components, SecDF. However, itsmolecular mechanisms remainunknown. In this study, we determined the crystal structure of Thermus thermophilusSecDF at 3.3 A resolutionand carried out a number of biochemical analysis based on the structural information.Finally, we propose a working hypothesis that SecDF functions as a membrane-integrated motor, powered by PMF, to achieveATP-independent full protein translocation ability of the Sec machinery